1. Field of the Invention
The present invention relates to an image forming lens system and an image pickup apparatus using the same.
2. Description of the Related Art
In a photography in which, a telephoto lens or an ultra-telephoto lens (hereinafter, appropriately let to be telephoto lens) is used, an effect of drawing a distant object or a small object in front of an eye of a photographer is achieved. Therefore, the telephoto lens has widely been used in photography of various scenes such as photography of sport scenes, photography of wild animals such as wild birds, and photography of astronomical bodies.
As a telephoto lens to be used for photography of such scenes, telephoto lenses disclosed in Japanese Patent Application Laid-open Publication Nos. Hei 9-236743 and Hei 11-160617 are available.
In the photography of abovementioned scenes, relative merits of mobility of an image pickup apparatus become important. Here, the mobility refers to an ease of carrying, a stability at the time of hand-held photography, and a rapidity of focusing speed. For making the mobility of an apparatus superior, an optical system having a small size and light weight is desirable. Moreover, a feature that an optical system is capable of focusing an object rapidly is an important feature that decides the relative merits of mobility.